Application of MVR System of Screw Steam Compressor in Alkali Recovery

Under the increasingly serious environmental pollution problem and the urgent need to save energy, many printing and dyeing enterprises have adopted the method of concentrating mercerized light alkali liquor for reuse to achieve energy saving and emission reduction. Adopt high temperature: 2014-04-02; revised draft date: 2014-05-13. Fund project: National Natural Science Foundation of China (51276134).

Author: Gao Lei (1990-) male, graduate. Contact: Xing Ziwen, professor. E-mailzwxingmail.xtu.edu. CN . Press steam directly to heat the waste liquid for single-effect concentration. The consumption of steam is large, the heat exchange process is intense, and the secondary steam generated by the concentration process is not used. Big.

The multi-effect treatment can effectively utilize the energy of the pre-effect secondary steam as the next-effect heating steam, but the exhaust steam generated by the final effect is directly condensed and discharged, and the raw steam is consumed while losing a large amount of low-temperature latent heat; the heat pump evaporation technology is capable of a large amount of The direct-use low-grade secondary steam is heated to a high-grade heat source by a compression process to heat an evaporation method of the working medium. In the evaporation process, both latent heat and compression work are recovered, so that the energy saving effect is remarkable. Heat pump evaporation is divided into two categories according to the principle: the boiling point r of mechanical compression is linear.

In addition to the above-mentioned loss of the effective heat transfer temperature difference caused by the increase of the boiling point of the lye, the liquid level should be included to cause the internal pressure of the liquid to be higher than the boiling point rise value A caused by the liquid surface, ", and in addition to the first effect The heat transfer temperature difference loss A,"' generated by the secondary steam flowing to the pressure drop caused by the flow resistance in the next process. In the falling film evaporator, A/small, the value of A/" is generally 0.51 °C. In the calculation process, A/+A/"=1C is taken, so the effective heat transfer temperature difference in the evaporation process is 7C, and the assumption is assumed. The isentropic efficiency of the three compressors is 0.75, and the pressure of the raw steam is 0.8 MPa. When the lye is preheated, it can reach the boiling point when entering the evaporator.

The rationality of the multi-effect system operating conditions is based on the assumption that each effect evaporation amount is in the three-effect system of Table 3. The secondary steam of the solution, the physical property of the solution, the first effect, the second effect, the third effect solution, the boiling point C / C heating steam temperature L /C secondary steam vaporization latent heat r / k solution enthalpy H / k.kg-1 Table 4 four-effect system in each effect secondary steam, solution physical properties first effect second effect third effect fourth effect solution boiling point /heating steam temperature T; / °C secondary steam vaporization latent heat solution enthalpy / k.kg1 3.2.3 four-effect evaporative heat pump system according to the double-effect medium compressor outlet pressure method to calculate the four-effect system outlet pressure id = .274MPa The boiling point of each effect solution, steam temperature, latent heat of vaporization value, and solution enthalpy value are summarized in Table 4. Calculated according to the assumed working conditions: r3c = 2223k/kg, = 501.8k/kg for four-effect treatment; The latent heat of vaporization does not meet the latent heat of vaporization under the assumed pressure. At this time, the evaporation pressure of the second effect can be appropriately reduced, and the pressure of the first effect can be appropriately increased to increase the amount of flash vapor of the condensate and increase the theoretical threshold of the secondary steam; Secondary steam as the next heating steam, the vapor pressure can not be reduced too much, when i1 =0.165MPa, /2=.135MPa, weight = 488.8k/kg when id=0.282MPa, and the concentration can basically meet the expected requirements.

3.3 The calculation results of the thermal performance of the system are based on. The evaporation system works 24 hours a day, 365 days a year, and the cost of the multi-effect evaporative heat pump system with different compressors is summarized in Table 6. Table 6 Economic comparison of multi-effect evaporative heat pump system Unit: Yuan/year project fixed cost operation cost Total compressor heat exchanger power consumption steam single-effect centrifugal double-effect Roots three-effect screw three-effect TVR four-effect screw four-effect TVR compares the data in Table 6 with the annual operating cost of simple processing, it can be seen that more Efficient evaporative heat pump system processing technology saves annual operating costs compared to simple purification treatment. Moreover, by comparing the double-effect and three-effect systems in Table 6, it can be found that the three-effect heat pump system composed of the screw steam compressor has high economical efficiency, compared with the double-effect system composed of the Roots model, although the compressor and The cost of the heat exchanger has increased, but the annual operating cost of the system is less, and the system can recover the increased equipment cost in one year. Of course, the relevant electricity price, equipment costs, etc. will change with the market price, and the payback period of the enterprise will also change.

4 Conclusion The multi-effect evaporative heat pump system was used to simulate the lye recovery of printing and dyeing enterprises, and the performance and economy of single-effect and multi-effect systems using centrifuge, Roots machine, screw machine and injector were compared and analyzed. , draw the following conclusions.

With the increase of the treatment efficiency of the multi-effect evaporative heat pump system, the total heat transfer amount gradually decreases, but the change is not large; the compressor power consumption gradually decreases, and the system performance coefficient CQP gradually increases.

The heat exchange area of ​​the system increases with the increase of the treatment efficiency, especially the area increase during the four-effect treatment. The main reason is that the increase of the boiling point of the solution leads to a decrease in the effective heat transfer temperature difference with the evaporation process.

Compared with the simple wastewater purification treatment method, the use of the heat pump system to evaporate the waste alkali liquor is economical; the system can recover the cost in one year. The ejector coefficient of the injector that draws secondary steam at atmospheric pressure is low, resulting in a lower TVR system economy than MVR, and the MVR system should be used as much as possible in the alkali recovery project.

The annual operating cost of the MVR system using the screw steam compressor is less than that of the MVR system consisting of the centrifuge and the Roots machine. The system can recover the increased equipment cost in one year, so it has advantages over other systems.

Excessive increase in efficiency will result in an increase in the cost of equipment including intermediate pumps, flash tanks, and heat exchangers. For printing and dyeing enterprises, compressor power consumption and equipment cost should be considered comprehensively. The use of screw steam compressor to achieve three-effect treatment can achieve better energy-saving and emission reduction effects.

Symbol Description f Complete lye mass flow, kg/s f4 initial lye mass flow, kg/s H-alkali solution enthalpy, k/kgH4 initial solution enthalpy, k/kg h water or water vapor enthalpy , k / kg h - isentropic compression of the actual enthalpy of the steam at the outlet, the enthalpy of saturated water at the k / kg outlet pressure, k / kg 屯 isentropic compression of the theoretical enthalpy of the steam at the outlet, k / kg K a An ejector coefficient, the heat transfer amount of the 1Q condenser, the second effect flash of kW 51, the boiling point of the kg/st alkali solution, the power of the CW-one compressor, the amount of water evaporated by the kW-one evaporator, Kg/s% compressor outlet steam mass flow, kg/s 1m-one flash-mixed second-effect heating steam mass flow, kg/s--one flash-mixed third-effect heated steam mass flow, kg/s The amount of steam generated by the ejector, kg/s Wn, the mass flow rate of the condensate injected into the compressor, the concentration of the finished liquid at kg/sx, the dimension is the initial lye concentration of 1 X4, and the dimension is 1

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